DMA (Direct Memory Access)

DMA (Direct Memory Access)

• In software control data transfer, processor executes a series of instructions to carry out data transfer. For each instruction execution fetch, decode and execute phases are required. Fig. 12.4.1 gives the flowchart to transfer data from memory to I/O device.

• Thus to carry out these tasks processor requires considerable time. So this method of data transfer is not suitable for large data transfers such as data transfer from magnetic disk or optical disk to memory.

Drawbacks in programmed I/O and interrupt driven I/O

1. The I/O transfer rate is limited by the speed with which the CPU can test and service a device.

2. The time that the CPU spends testing I/O device status and executing a number of instructions for I/O data transfers can often be better spent on other processing tasks.

• To overcome above drawbacks an alternative technique, hardware controlled data transfer can be used.

 

1. DMA Operation

• DMA controlled data transfer is used for large data transfer. For example to read bulk amount data from disk to main memory.

• To read a block of data from the disk processor sends a series of commands to the disk controller device telling it to search and read the desired block of data from the disk.

• When disk controller is ready to transfer first byte of data from disk, it sends DMA request DRQ signal to the DMA controller.

• Then DMA controller sends a hold request HRQ, signal to the processor HOLD input. The processor responds this HOLD signal by floating its buses and sending out a hold acknowledge signal HLDA, to the DMA controller.

• When the DMA controller receives the HLDA signal, it takes the control of system bus.

• When DMA controller gets control of the buses, it sends the memory address where the first byte of data from the disk is to be written. It also sends a DMA acknowledge, DACK signal to the disk controller device telling it to get ready to output the byte.

• Finally, it asserts both the I/O read and memory write signals on the control bus. Asserting the I/O read signal enables the disk controller to output the byte of data from the disk on the data bus and asserting the memory write signal enables the addressed memory to accept data from the data bus. In this technique data is transferred directly from the disk controller to the memory location without passing through the processor or the DMA controller.

• Thus, the CPU is involved only at the beginning and end of the transfer.

• After completion of data transfer, the HOLD signal is deasserted to give control of all buses back to the processor.

Fig. 12.4.2 shows the interaction between processor and DMA discussed above.

 

2. Comparison of I/O Program Controlled Transfer and DMA Transfer

I/O Program controlled transfer

1. It is software controlled data transfer

2. Data transfer speed is low.

3. CPU is involved in the transfer.

4. Extra hardware is not required.

5. During data transfer data is routed through processor.

DMA transfer

1. Hardware controlled data transfer.

2. Data transfer speed is high.

3. CPU is not involved in the transfer.

4. DMA controller is required to data transfer.

5. During data transfer data does not routed through processor.

 

3. DMA Block Diagram

• For performing the DMA operation, the basic blocks required in a DMA channel/controller are shown in Fig. 12.4.3.

• DMA controller communicates with the CPU via the data bus and control lines.

• The registers in DMA are selected by the CPU through the address bus by enabling the DS (DMA select) and RS (Register select) inputs.

• The RD (Read) and WR (write) inputs are bidirectional.

• When the BG (bus grant) input is 0, the CPU can communicate with the DMA registers through the data bus to read from or write the DMA registers RD and WR signals are input signals for DMA.

• When BG = 1, the CPU has relinquished the buses and the DMA can communicate directly with the memory by specifying an address in the address bus and activating the RD or WR signals (RD and WR are now output signals for DMA). DMA consists of data count, data register, address register and control logic.

• Data counter register stores the number which gives the number data transfers to be done in one DMA cycle. It is automatically decremented after each word transfer.

• Data register acts as buffer whereas address register initially holds the starting address of the device. Actually, it stores the address of the next word to be transferred. It is automatically incremented or decremented after each word transfer.

• After each transfer, data counter is tested for zero. When the data count reaches zero, the DMA transfer halts.

• The DMA controller is normally provided with an interrupts capability, in which case it sends an interrupt to processor to signal the end of the I/O data transfer.

4. Data Transfer Modes

• DMA controller transfers data in one of the following three modes :

■ Single transfer mode (cycle stealing)

■ Block transfer mode

■ Demand or burst transfer mode

Single transfer mode (Cycle stealing mode)

In this mode device can make only one transfer (byte or word). After each transfer DMAC gives the control of all buses to the processor. Due to this processor can have access to the buses on a regular basis.

It allows the DMAC to time share the buses with the processor, hence this mode is most commonly used.

The operation of the DMA in a single transfer mode is as given below :

1. I/O device asserts DRQ line when it is ready to transfer data.

2. The DMAC asserts HLDA line to request use of the buses from the processor.

3. The processor asserts HLDA, granting the control of buses to the DMAC.

4. The DMAC asserts  to the requesting I/O device and executes DMA bus cycle, resulting data transfer.

5. I/O device deasserts its DRQ after data transfer of one byte or word.

6. DMA deasserts  line.

7. The word/byte transfer count is decremented and the memory address is incremented.

8. The HOLD line is deasserted to give control of all buses back to the processor.

9. HOLD signal is reasserted to request the use of buses when I/O device is ready to transfer another byte or word. The same process is then repeated until the last transfer.

10. When the transfer count is exhausted, terminal count is generated to indicate the end of the transfer.

Block transfer mode

In this mode device can make number of transfers as programmed in the word count register. After each transfer word count is decremented by 1 and the address is decremented or incremented by 1. The DMA transfer is continued until the word count "rolls over" from zero to FFFFH, a Terminal Count (TC) or an external END of Process () is encountered. Block transfer mode is used when the DMAC needs to transfer a block of data.

The operation of DMA in block transfer mode is as given below :

1. I/O device asserts DRQ line when it is ready to transfer data.

2. The DMAC asserts HLDA line to request use of the buses from the microprocessor.

3. The microprocessor asserts HLDA, granting the control of buses to the DMAC.

4. The DMAC asserts  to the requesting I/O device and executes DMA bus cycle, resulting data transfer.

5. I/O device deasserts its DRQ after data transfer of one byte or word.

6. DMA deasserts  line.

7. The word/byte transfer count is decremented and the memory address is incremented.

8. When the transfer count is exhausted, the data transfer is not complete and the DMAC waits for another DMA request from the I/O device, indicating that it has another byte or word to transfer. When DMAC receives DMA request steps through are repeated.

9. If the transfer count is not exhausted, the data transfer is complete then DMAC deasserts the HOLD to tell the microprocessor that it no longer needs the buses.

10. Microprocessor then deasserts the HLDA signal to tell the DMAC that it has resumed control of the buses.

Demand transfer mode

In this mode the device is programmed to continue making transfers until a TC or external  is encountered or until DREQ goes inactive.

The operation of DMA in demand transfer mode is as given below :

1. I/O device asserts DRQ line when it is ready to transfer data.

2. The DMAC asserts HLDA line to request use of the buses from the microprocessor.

3. The microprocessor asserts HLDA, granting the control of buses to the DMAC.

4. The DMAC asserts  to the requesting I/O device and executes DMA bus cycle, resulting data transfer.

5. I/O device deasserts its DRQ after data transfer of one byte or word.

6. DMA deasserts  line.

7. The word/byte transfer count is decremented and the memory address is incremented.

8. The DMAC continues to execute transfer cycles until the I/O device deasserts DRQ indicating its inability to continue delivering data. The DMAC deasserts HOLD signal, giving the buses back to microprocessor. It also deasserts .

9. I/O device can re–initiate demand transfer by reasserting DRQ signal.

10. Transfer continues in this way until the transfer count has been exhausted.

The flowcharts in Fig. 12.4.4 summarized the three data transfer modes of DMA.

 

5. Use of DMA in a Computer System

• Fig. 12.4.5 (a) shows the use of DMA in a computer system.

• The DMA is used to connect a high–speed network to the computer bus. The DMA control handles the data transfer between high–speed network and the computer system.

• It is also used to transfer data between processor and floppy disk with the help of floppy disk controller.

• Let us see how DMA controller does the data transfer between floppy disk and the processor. The Fig. 8.10.5 (b) shows the interface required for such transfer.

• The sequence of events that takes place during the data transfer are as follows :

• When processor needs some data from the disk, it sends a series of command words to registers inside the floppy disk controller.

• The floppy disk controller then proceeds to find the specified track and sector on the disk.

• In the mean while processor loads the DMA data counter and address register. The data counter is loaded with count equal to the number of bytes to be transferred to or from the memory. The address register is loaded with the starting address of the memory.

• When the controller reads the first byte of data from a sector, it sends DMA request, DRQ signal to the DMA controller. DMA request sends a hold request signal to the HOLD input of the processor.

• The processor then floats its buses and sends a hold–acknowledge signal to the DMA controller.

• The DMA controller then sends out the first transfer address on the bus and asserts the   input of the FDC to tell it that the DMA transfer is underway.

• When the number of bytes specified in the DMA controller initialization has been transferred, the DMA controller asserts the  (end of process) signal, which is connected to the TC (Terminal Count) input of the FDC.

• This causes FDC to generate interrupt signal to tell the processor that the requested block of data has been read from disk to a buffer in memory.

• Similar process is required to write data into the disk.

 

Review Questions

1. State the advantages of DMA over the other modes of I/O transfer.

2. What is DMA operation? State its advantages.

3. Why do we need DMA ?

4. Write note on : DMA

5. Explain in detail about Direct Memory Access methods (DMA).

6. Discuss the DMA driven data transfer technique.

7. Comparison between I/O program controlled transfer and DMA transfer.

8. With a neat sketch explain the working principle of DMA.

9. Explain the DMA.

10. What are the necessary operations needed to start an I/O operation using DMA?

11. Explain how DMA transfer is accomplished with a neat diagram.

12. Explain the steps involved in the DMA operation.

13. Discuss direct memory access in detail.

14. What is meant by Direct Memory Access? Explain the use of DMA controllers in a computer system.

15. Draw the typical block diagram of a DMA controller and explain how it is used for direct data transfer between memory and peripherals ?

16. Explain about DMA controller, with the help of a block diagram.